🐛 fix(models): retry OpenRouter error responses

[bebiksior]

Occasionally, OpenRouter with GPT models returns a response containing an error:

{
  "error": {
    "message": "This content was flagged for possible cybersecurity risk...",
    "code": 502
  }
}

Because the response has no choices, mini-swe-agent throws KeyError: 'choices' outside its retry loop.

This change validates the response inside the retry boundary and raises OpenRouterAPIError, allowing the existing retry policy to handle it.